Programmable Digital Frequency Meter LCD 0.1 - 60 MHz

Ref: FREQ/60MHz

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Programmable Digital Frequency Meter LCD 0.1 - 60 MHz

This is a 60 MHz frequency meter / counter for measuring frequency from 10 Hz to 60 MHz with 10 Hz resolution. The meter provides very stable readings and has excellent input sensitivity thanks to the on-board amplifier and the TTL converter, so you can even measure weak signals from crystal oscillators. With the addition of prescaller it is possible to measure the frequency of 1 GHz and above.

The basic idea comes from the Microchip AN592 application note: "Frequency counter using PIC16C5x" where you can find a simple software that implements a frequency counter using a PIC microcontroller. I wrote software specifically designed to improve the resolution of meters, to manage the IF Mode and value via an operation menu, to decode and change the reading frequency on an LCD display. The result was a simple and effective device.

Circuit / Diagram
The electrical diagram is very simple, given that most of the functions are implemented by the microprocessor. Only one amplification step was needed to increase the input signal level from 200-300 mV pp to approximately 3 times pp. A self-biased common-emitter amplifier produces a pseudo-TTL driving signal. The 10uH inductor in the collector lead helps extend the high frequency response. Any "fast" NPN transistor should be suitable. I used a BFR91, but you can replace a shortened transistor from an old TV tuner or VHF receiver.
The amplifier's quiescent Vce is set to 1.8 to 2.2 volts with the resistor marked * on the diagram. It is rated 10K, but you may need to change it. The collector voltage is applied to the PIC's counter/timer via a 470 ohm series resistor. The PIC is able to shorten this signal to ground via an internal pull-down transistor to turn off the counting

The PIC implements a 32-bit counter, partly in internal hardware and partly in software. Counting is enabled by turning off the internal drive transistor for "exactly" 0.4 seconds. At the end of this time, the PIC divides the count by 4, then adds or subtracts the appropriate IF frequency to obtain the actual frequency. The resulting count is converted into printable characters and delivered
To the display.

Calibration: Before use
       Before the heart rate meter works properly, it must be calibrated. This can be as simple as connecting a known frequency source and adjusting the trimmer capacitor so that the correct value is displayed. If the displayed frequency cannot be adjusted, a "coarse calibration" is required. This involves booting from shutdown. Pin 10 is connected to ground and is therefore turned on (and held down). The PIC will measure and display the input frequency, followed by the letters CAL. If it is not possible to adjust the indicated frequency to the correct value (by adjusting the 33 pF trimmer), it is possible to make coarse adjustments by briefly connecting pin 12 or pin 13 to ground. It may take several attempts, as the program checks these pins only once per measurement (0.4 seconds). Once you are satisfied with the adjustment, remove ground from pin 10 (while power is still applied). In this way the PIC stores the calibration in internal non-volatile memory.


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